1. Filed of the Invention
The present invention relates to an image reader for reading an image using a plurality of image sensors.
2. Related Background Art
As an image sensor which has been conventionally used to read an image, an image sensor, in which linear image sensors are provided with a stripe type of color filter and a color separation signal is time-shared to be dot-sequentially read, has been known.
When reading an original, for example, a size A4 297 mm in longitudinal width, a silicon crystal type of image sensor is suitable to read at high speeds. In the case of the silicon crystal type image sensor, however, it is difficult to manufacture a longitudinal type sensor with one chip because of some restrictions in the manufacture, and therefore, a linear sensor is composed of a plurality of chips by exerting ingenuity in the physical placement.
FIG. 9 is a schematic diagram showing the light receiving surface of an image sensor in which linear sensor chips 100 to 104 are arranged in zigzag fashion on a substrate 105. In the linear sensor chips 100 to 104, a plurality of light receiving elements consisting of amorphous silicon, etc. have been lined up. In FIG. 9, each linear sensor chip, 100 to 104, is scanned in the arrow MS direction, and outputs an analog signal, which is synchronized with a specified clock pulse and corresponds to the intensity of the received light, to an output, OUT, for each pixel.
Also, on actually reading an original image, this multi-chip CCD sensor travels relative to the original in a direction (arrow SS direction) perpendicular to the main scanning direction of the linear sensor. Accordingly, the linear sensor chips 101 and 103 read and scan the original a specified line ahead of the linear sensor chips 100, 102 and 104.
FIG. 10 shows an example of a configuration of an original reader using an image sensor shown in FIG. 9, and numeral 207 is an image sensor shown in FIG. 9. In FIG. 10, numeral 218 is an original having an image to be read, numeral 220 is an irradiation lamp, numeral 221 is a reflective mirror, numeral 217 is an original loading glass (an original loading stand), numeral 202 is a standard plate providing a reference level for a read signal, and numeral 223 is a rod lens array for focusing reflecting light from an original 218 placed on the original loading glass 217 onto the image sensor 207. A scanning unit 210 is mounted with sliding members 203 and 204 at the bottom, which slide on the surface of two rails 201.
A wire 215 drives the scanning unit 210, and is secured to members provided on both sides of the scanning unit 210 with fitments. A pulley 211 drives the wire 215, and is secured to a shaft 225. An idle pulley 214 forms a counterpart to the pulley 211, and is pulled by a spring 236 in such a direction as to apply a tension to a driving wire 215. Numeral 227 is a motor, numeral 212 is a motor pulley, and a belt 213 transmits a rotary motion of a motor pulley 212 to the pulley 211.
Numeral 243 is a CCD driver and amplifier circuit, and a signal line 241 transmits a signal from the image sensor 207 to the amplifier circuit 243. A processing unit 216 performs the functions of memory, operation, output, etc., and a signal line 242 transmits a signal from an amplifier circuit 243 to the processing unit 216.
A rotary motion of the motor 227 is converted to a linear motion of the scanning unit 210 through the motor pulley 212, the belt 213, the pulley 211 and the wire 215. The scanning unit 210 slides (reciprocates) on the surface of the rails 201 in the arrow direction in FIG. 10 through sliding members 203 and 204 by forward or reverse rotations of the motor 227. At this time, irradiation lamp 220, reflective mirror 221, rod lens array 223, image sensor 207, signal line 241, and CCD driver and amplifier circuit 243 perform the same motion as the scanning unit 210 because all of them are mounted on the scanning unit 210.
The original 218 is irradiated by the irradiation lamp 220, and its reflected light is image-formed on the image sensor 207 through the rod lens array 223. The image sensor 207 outputs an analog signal corresponding to this image-formed image. The analog signal is transmitted to the amplifier circuit 243 through the signal line 241, and is transmitted to the processing unit 216 through the signal line 242 after it is converted into a specified level of analog signal by the amplifier circuit 243.
The moment it scans the original for each line as mentioned above, the scanning unit 210 travels (subscans) in the arrow ss direction in FIG. 10 to scan the entire desired area of the original.
In the above-mentioned configuration, the temperature at the image sensor 207 rises due to heat generation in the image sensor itself because the load capacity of the image sensor 207 is great during driving and the driving frequency therefor should be made higher to read at high speeds. In addition, since such heat release elements as the irradiation lamp 220, the CCD driver and amplifier circuit 243 are located near the image sensor 207, the temperature at the image sensor 207 also rises due to their heat generation.
Such a rise in temperature of the image sensor 207 causes fluctuations in the output level of the image sensor 207, and specially a change in dark current in the image sensor 207.
Even in the circuit unit of the processing unit 216 for processing an output signal from the image sensor 207, fluctuations in the level of the processing output signal occur for various reasons such as environmental temperature and variations with time.
To eliminate the effects of the change in dark current due to a rise in temperature of the image sensor 207 and variations in the level of a dark output unit due to the circuit configuration and the like, a clamping circuit was provided which detects the signal level of the dark output unit of the image sensor 207 once per horizontal scan to direct-current restore the detected signal level at a specified potential.
However, the reflected light leaks from the edge of the cut area of the image sensor chip into the dark output unit in the image sensor to change the level. In the clamping circuit, there was an inconvenience that the significant image level relatively changes because the dark output unit, in which the level has changed, is fixed to a specified potential.
Also, in a configuration in which an image is read using a plurality of sensor chips as shown in FIGS. 9 and 10 mentioned above, when a bright light corresponding to white in an original image comes to an area corresponding to the dark output unit of a chip having the image sensor 207, there was a defect that the density changes in the horizontal scanning direction, and a difference in density level occurs between each image corresponding to each chip.